1. Field of the Invention
The present invention relates to a manufacturing method of a thin film transistor and a display device.
2. Description of the Related Art
In recent years, thin film transistors that are formed using a semiconductor thin film having a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attentions. Thin film transistors are widely used for electronic devices such as integrated circuits (ICs) and electro-optical devices. Thin film transistors are rapidly developed as switching elements, particularly for image display devices typified by liquid crystal display devices, electroluminescence (EL) display devices, and the like. In an active matrix liquid crystal display device, a voltage is applied between a pixel electrode connected to a selected switching element and an opposite electrode corresponding to the pixel electrode, and thus, a liquid crystal layer disposed between the pixel electrode and the opposite electrode is modulated optically. The optical modulation can be recognized as a display pattern by an observer. An active matrix liquid crystal display device here means a liquid crystal display device which employs a method in which a display pattern is formed on a screen by driving pixel electrodes arranged in matrix using switching elements. An active matrix EL display device means an EL display device which employs a method in which a display pattern is formed on a screen by driving pixels arranged in matrix using switching elements.
The application range of active matrix display devices is expanding and demand for larger screen size, higher definition, and higher aperture ratio is increasing. In addition, it is demanded that the active matrix display devices have high reliability and that a production method of the active matrix display devices offers a high fabrication yield and reduces production cost. As a method for increasing a fabrication yield and reducing production cost, simplification of the process can be given.
In active matrix display devices, thin film transistors are mainly used as switching elements. In manufacturing thin film transistors, reduction in the number of photomasks used in photolithography is important for simplification of the process. For example, when one photomask is added, the following steps are further needed: resist application, prebaking, light exposure, development, postbaking, and the like and, moreover, steps before and after the aforementioned steps, such as film formation, etching, resist removal, cleaning, drying, and the like. The number of steps is significantly increased only by adding one photomask in the manufacturing process. Therefore, many techniques for reducing the number of photomasks in a manufacturing process have been developed.
Thin film transistors are broadly classified into top gate thin film transistors in which a channel formation region is provided below a gate electrode, and bottom gate thin film transistors in which a channel formation region is provided above a gate electrode. It is known that the number of photomasks used in a process for manufacturing a bottom gate thin film transistor is smaller than that in a process for manufacturing a top gate thin film transistor. A bottom gate thin film transistor is generally manufactured using three photomasks.
Many conventional techniques for reducing the number of photomasks use a complicated technique such as backside light exposure, resist reflow, or a lift-off method, which requires a special apparatus. The fabrication yield or the like can decrease due to various problems caused by usage of such a complicated technique. Moreover, electric characteristics of thin film transistors have to be sacrificed in many cases.
As a typical way for reducing the number of photomasks in a manufacturing process of a thin film transistor, a technique using a multi-tone mask (referred to as a half-tone mask or a gray-tone mask) is widely known. As a technique for reducing the number of manufacturing steps by using a multi-tone mask, Patent Document 1 is disclosed, for example.